I have infinite mode and know to get the real mega buster you need zombie..

Heres can surive longer get the health books in Enterance plaza, wonder plaza and survival book in North Plaza next to hardware store.You be set with those books, then get the wine in food court and North plaza, and go outside get the red car go M. Tunnels go left to working ice cream truck use it to kill carlito.
Use that ice cream truck if breaks go to butcher then go back out to recive a new ice cream truck.Keep doing this until have one bar left then drink a wine or milk to be full health.Do this until have no more wine then you kill the surivers.

When I did geniciod it took me three days in infini mode

The way to do this is play the game in 72 hour mode but do not do the story line or save anyone. Go to where the car and bike are, take the car and kill a few as you drive the car to the tunnel go down the tunnel and go left kill them as you go when you get to the end of the tunnel keep driving around killing until your car blows up then jump in the van and do the same in the opposite direction till you come to the exit where you go back to the car park and get the car and do the same again and again till you kill 53.594 zombies it will atke aboat 4 hours, that is human hours not game hours

This is going to take Along time to do but you have to drive in the Maintenence tunnel untill about day 2 just driving around killing Zombies and totally forget about the missions because you can't get 53,000 some Zombies killed doing the missions and trying to kill them so just find the maintenence key(once enturing the tunnel from the parking lot take a right, then left, then right and walk in the little storage room and it will be on the right side of the wall in the back.) and in Paradice Plaza theres a restroom area that you can save in and then a door leading to the Tunnel a easy place to save and easy place to find a car because their will spawn a luxery white car when you walk out.